adding parallel threads

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/sat/sat_par.cpp

@@ -0,0 +1,45 @@
+/*++
+Copyright (c) 2017 Microsoft Corporation
+
+Module Name:
+
+    sat_par.cpp
+
+Abstract:
+
+    Utilities for parallel SAT solving.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2017-1-29.
+
+Revision History:
+
+--*/
+#include "sat_par.h"
+
+
+namespace sat {
+
+    par::par() {}
+
+    void par::exchange(literal_vector const& in, unsigned& limit, literal_vector& out) {
+        #pragma omp critical (par_solver)
+        {
+            if (limit < m_units.size()) {
+                // this might repeat some literals.
+                out.append(m_units.size() - limit, m_units.c_ptr() + limit);
+            }
+            for (unsigned i = 0; i < in.size(); ++i) {
+                literal lit = in[i];
+                if (!m_unit_set.contains(lit.index())) {
+                    m_unit_set.insert(lit.index());
+                    m_units.push_back(lit);
+                }
+            }
+            limit = m_units.size();
+        }
+    }
+    
+};
+